1. Field of the Invention
The present invention relates to a method for manufacturing a dental restoration.
2. Description of the Related Art
Different methods are known in the art for manufacturing a dental restoration, wherein these methods are mainly determined by the type of materials used.
Ceramic materials have been successfully applied for many years in dental restorations. The two most important functions of restorative ceramics are aesthetics and strength. However, most dental ceramics do not fulfil both functions. Different porcelains and glass ceramics are used for aesthetic applications. These have a natural tooth colour and translucence but have a relatively low three-point bending strength (50-200 MPa).
Higher-strength materials have been developed as basic core material. These materials have a three-point bending strength of 150 to 500 MPa, but are generally too opaque and must be combined with translucent porcelain veneered thereon. The fracture toughness however remains low compared to metal alloys, whereby these ceramics are susceptible to production errors and stress concentrations, such as occur when there is a less than optimal fit between prepared tooth and the restorative ceramic.
Medically pure yttrium-stabilized tetragonal zirconium oxide (Y-TZP) is characterized by both aesthetics and a high three-point bending strength, higher than 1000 MPa, with a great toughness and an excellent resistance to slow crack development. Zirconium oxide has heretofore been applied very successfully in orthopaedics as hip-joint ball. It has also been very successfully introduced in dentistry for root pins. It has further been found to have a very low susceptibility to dental plaque. However, zirconium oxide has a white colour not suitable for dental restorations and must be coloured with a colour corresponding with the tooth colours.
In DE 4207179 A1 (1992) Yoshida describes a method of colouring orthodontic zirconium oxide components by adding a mixture of erbium-, praseodymium-, iron- and zinc oxides. Some additives, such as zinc, result in serious degradation of the physical properties of the sintered zirconium oxide. In DE 19938143 A1 erbium-, iron- and manganese oxide are mainly applied as colouring oxides.
Effect of suitable colour additives to sintered Y-TZP zirconium oxide
Colour additiveFormEffective colourNoteIronFe2O3BrownConcentration lowerthan 1% by weightErbiumEr2O3Light violetForms solid solutionwith ZrO2PraseodymiumPr2O3Deep yellowForms solid solutionwith ZrO2
These are first dissolved in hydrochloric acid and added to zirconium oxide and admixtures dissolved in hydrochloric acid. By hydrolysis with ammonia, annealing of the deposition and fine-grinding, an homogeneously coloured zirconium oxide powder is obtained which can be further processed. This takes place by mixing the powder with binders (for instance 2% by weight polyvinyl alcohol and 0.15% by weight oleic acid), and making this into a geometrical shape by pressing. The binder is burnt out for 0.5 to 2 hours at a temperature of 850-1000° C. A dental restoration, enlarged by the sintering shrinkage factor, is then cut out by means of a CAD/CAM system and the product is sintered to increase density at 1300-1500° C. for 2-4 hours. The result is a monochrome coloured restoration.
It is however desired in dentistry to colour the restoration locally. The base structure of a crown, in the form of a cap, is thus coloured slightly browner at the bottom, the so-called shoulder portion, and lighter yellow in the sections in the cutting edge area.
At the moment the restorations are further produced in automatic manner with fairly traditional cutting technique, with proportionately high material loss. With the use of zirconium oxide the price of the material is considerable and a net-shape production method would be desirable. Accurate colouring with locally occurring variations is complex here, if not impossible.
Up to the present there has been no possibility of making restorations using rapid prototyping or even in automated manner which have locally different properties or colours. In addition to the absence of an experimental configuration, there has also been no possibility of locally colouring product files.
In order to obviate or at least reduce the above stated and other problems and drawbacks of the known art, a new method for manufacturing a dental restoration is provided according to the present invention.